[R] sampling rows with values never sampled before

[C W]

Hello R list,

I am have question about sampling unique coordinate values.

Here's how my data looks like

> dat <- cbind(x1 = rep(1:5, 3), x2 = rep(c(3.7, 2.9, 5.2), each=5))
> dat
      x1  x2
 [1,]  1 3.7
 [2,]  2 3.7
 [3,]  3 3.7
 [4,]  4 3.7
 [5,]  5 3.7
 [6,]  1 2.9
 [7,]  2 2.9
 [8,]  3 2.9
 [9,]  4 2.9
[10,]  5 2.9
[11,]  1 5.2
[12,]  2 5.2
[13,]  3 5.2
[14,]  4 5.2
[15,]  5 5.2


If I sampled (1, 3.7), then, I don't want (1, 2.9) or (2, 3.7).

I want to avoid either the first or second coordinate repeated.  It leads
to undefined matrix inversion.

I thought of using sampling(), but not sure about applying it to a data
frame.

Thanks in advance,

Mike

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